/tests/compute/test_subgraph.py

import numpy as np
import networkx as nx
import unittest
import scipy.sparse as ssp
import pytest

import dgl
import backend as F
from test_utils import parametrize_idtype

D = 5

def generate_graph(grad=False, add_data=True):
    g = dgl.DGLGraph().to(F.ctx())
    g.add_nodes(10)
    # create a graph where 0 is the source and 9 is the sink
    for i in range(1, 9):
        g.add_edge(0, i)
        g.add_edge(i, 9)
    # add a back flow from 9 to 0
    g.add_edge(9, 0)
    if add_data:
        ncol = F.randn((10, D))
        ecol = F.randn((17, D))
        if grad:
            ncol = F.attach_grad(ncol)
            ecol = F.attach_grad(ecol)
        g.ndata['h'] = ncol
        g.edata['l'] = ecol
    return g

def test_edge_subgraph():
    # Test when the graph has no node data and edge data.
    g = generate_graph(add_data=False)
    eid = [0, 2, 3, 6, 7, 9]

    # relabel=True
    sg = g.edge_subgraph(eid)
    assert F.array_equal(sg.ndata[dgl.NID], F.tensor([0, 2, 4, 5, 1, 9], g.idtype))
    assert F.array_equal(sg.edata[dgl.EID], F.tensor(eid, g.idtype))
    sg.ndata['h'] = F.arange(0, sg.number_of_nodes())
    sg.edata['h'] = F.arange(0, sg.number_of_edges())

    # relabel=False
    sg = g.edge_subgraph(eid, relabel_nodes=False)
    assert g.number_of_nodes() == sg.number_of_nodes()
    assert F.array_equal(sg.edata[dgl.EID], F.tensor(eid, g.idtype))
    sg.ndata['h'] = F.arange(0, sg.number_of_nodes())
    sg.edata['h'] = F.arange(0, sg.number_of_edges())

def test_subgraph():
    g = generate_graph()
    h = g.ndata['h']
    l = g.edata['l']
    nid = [0, 2, 3, 6, 7, 9]
    sg = g.subgraph(nid)
    eid = {2, 3, 4, 5, 10, 11, 12, 13, 16}
    assert set(F.asnumpy(sg.edata[dgl.EID])) == eid
    eid = sg.edata[dgl.EID]
    # the subgraph is empty initially except for NID/EID field
    assert len(sg.ndata) == 2
    assert len(sg.edata) == 2
    sh = sg.ndata['h']
    assert F.allclose(F.gather_row(h, F.tensor(nid)), sh)
    '''
    s, d, eid
    0, 1, 0
    1, 9, 1
    0, 2, 2    1
    2, 9, 3    1
    0, 3, 4    1
    3, 9, 5    1
    0, 4, 6
    4, 9, 7
    0, 5, 8
    5, 9, 9       3
    0, 6, 10   1
    6, 9, 11   1  3
    0, 7, 12   1
    7, 9, 13   1  3
    0, 8, 14
    8, 9, 15      3
    9, 0, 16   1
    '''
    assert F.allclose(F.gather_row(l, eid), sg.edata['l'])
    # update the node/edge features on the subgraph should NOT
    # reflect to the parent graph.
    sg.ndata['h'] = F.zeros((6, D))
    assert F.allclose(h, g.ndata['h'])

def _test_map_to_subgraph():
    g = dgl.DGLGraph()
    g.add_nodes(10)
    g.add_edges(F.arange(0, 9), F.arange(1, 10))
    h = g.subgraph([0, 1, 2, 5, 8])
    v = h.map_to_subgraph_nid([0, 8, 2])
    assert np.array_equal(F.asnumpy(v), np.array([0, 4, 2]))

def create_test_heterograph(idtype):
    # test heterograph from the docstring, plus a user -- wishes -- game relation
    # 3 users, 2 games, 2 developers
    # metagraph:
    #    ('user', 'follows', 'user'),
    #    ('user', 'plays', 'game'),
    #    ('user', 'wishes', 'game'),
    #    ('developer', 'develops', 'game')])

    g = dgl.heterograph({
        ('user', 'follows', 'user'): ([0, 1], [1, 2]),
        ('user', 'plays', 'game'): ([0, 1, 2, 1], [0, 0, 1, 1]),
        ('user', 'wishes', 'game'): ([0, 2], [1, 0]),
        ('developer', 'develops', 'game'): ([0, 1], [0, 1])
    }, idtype=idtype, device=F.ctx())
    for etype in g.etypes:
        g.edges[etype].data['weight'] = F.randn((g.num_edges(etype),))
    assert g.idtype == idtype
    assert g.device == F.ctx()
    return g

@unittest.skipIf(dgl.backend.backend_name == "mxnet", reason="MXNet doesn't support bool tensor")
@parametrize_idtype
def test_subgraph_mask(idtype):
    g = create_test_heterograph(idtype)
    g_graph = g['follows']
    g_bipartite = g['plays']

    x = F.randn((3, 5))
    y = F.randn((2, 4))
    g.nodes['user'].data['h'] = x
    g.edges['follows'].data['h'] = y

    def _check_subgraph(g, sg):
        assert sg.idtype == g.idtype
        assert sg.device == g.device
        assert sg.ntypes == g.ntypes
        assert sg.etypes == g.etypes
        assert sg.canonical_etypes == g.canonical_etypes
        assert F.array_equal(F.tensor(sg.nodes['user'].data[dgl.NID]),
                             F.tensor([1, 2], idtype))
        assert F.array_equal(F.tensor(sg.nodes['game'].data[dgl.NID]),
                             F.tensor([0], idtype))
        assert F.array_equal(F.tensor(sg.edges['follows'].data[dgl.EID]),
                             F.tensor([1], idtype))
        assert F.array_equal(F.tensor(sg.edges['plays'].data[dgl.EID]),
                             F.tensor([1], idtype))
        assert F.array_equal(F.tensor(sg.edges['wishes'].data[dgl.EID]),
                             F.tensor([1], idtype))
        assert sg.number_of_nodes('developer') == 0
        assert sg.number_of_edges('develops') == 0
        assert F.array_equal(sg.nodes['user'].data['h'], g.nodes['user'].data['h'][1:3])
        assert F.array_equal(sg.edges['follows'].data['h'], g.edges['follows'].data['h'][1:2])

    sg1 = g.subgraph({'user': F.tensor([False, True, True], dtype=F.bool),
                      'game': F.tensor([True, False, False, False], dtype=F.bool)})
    _check_subgraph(g, sg1)
    sg2 = g.edge_subgraph({'follows': F.tensor([False, True], dtype=F.bool),
                           'plays': F.tensor([False, True, False, False], dtype=F.bool),
                           'wishes': F.tensor([False, True], dtype=F.bool)})
    _check_subgraph(g, sg2)

@parametrize_idtype
def test_subgraph1(idtype):
    g = create_test_heterograph(idtype)
    g_graph = g['follows']
    g_bipartite = g['plays']

    x = F.randn((3, 5))
    y = F.randn((2, 4))
    g.nodes['user'].data['h'] = x
    g.edges['follows'].data['h'] = y

    def _check_subgraph(g, sg):
        assert sg.idtype == g.idtype
        assert sg.device == g.device
        assert sg.ntypes == g.ntypes
        assert sg.etypes == g.etypes
        assert sg.canonical_etypes == g.canonical_etypes
        assert F.array_equal(F.tensor(sg.nodes['user'].data[dgl.NID]),
                             F.tensor([1, 2], g.idtype))
        assert F.array_equal(F.tensor(sg.nodes['game'].data[dgl.NID]),
                             F.tensor([0], g.idtype))
        assert F.array_equal(F.tensor(sg.edges['follows'].data[dgl.EID]),
                             F.tensor([1], g.idtype))
        assert F.array_equal(F.tensor(sg.edges['plays'].data[dgl.EID]),
                             F.tensor([1], g.idtype))
        assert F.array_equal(F.tensor(sg.edges['wishes'].data[dgl.EID]),
                             F.tensor([1], g.idtype))
        assert sg.number_of_nodes('developer') == 0
        assert sg.number_of_edges('develops') == 0
        assert F.array_equal(sg.nodes['user'].data['h'], g.nodes['user'].data['h'][1:3])
        assert F.array_equal(sg.edges['follows'].data['h'], g.edges['follows'].data['h'][1:2])

    sg1 = g.subgraph({'user': [1, 2], 'game': [0]})
    _check_subgraph(g, sg1)
    sg2 = g.edge_subgraph({'follows': [1], 'plays': [1], 'wishes': [1]})
    _check_subgraph(g, sg2)

    # backend tensor input
    sg1 = g.subgraph({'user': F.tensor([1, 2], dtype=idtype),
                      'game': F.tensor([0], dtype=idtype)})
    _check_subgraph(g, sg1)
    sg2 = g.edge_subgraph({'follows': F.tensor([1], dtype=idtype),
                           'plays': F.tensor([1], dtype=idtype),
                           'wishes': F.tensor([1], dtype=idtype)})
    _check_subgraph(g, sg2)

    # numpy input
    sg1 = g.subgraph({'user': np.array([1, 2]),
                      'game': np.array([0])})
    _check_subgraph(g, sg1)
    sg2 = g.edge_subgraph({'follows': np.array([1]),
                           'plays': np.array([1]),
                           'wishes': np.array([1])})
    _check_subgraph(g, sg2)

    def _check_subgraph_single_ntype(g, sg, preserve_nodes=False):
        assert sg.idtype == g.idtype
        assert sg.device == g.device
        assert sg.ntypes == g.ntypes
        assert sg.etypes == g.etypes
        assert sg.canonical_etypes == g.canonical_etypes

        if not preserve_nodes:
            assert F.array_equal(F.tensor(sg.nodes['user'].data[dgl.NID]),
                                 F.tensor([1, 2], g.idtype))
        else:
            for ntype in sg.ntypes:
                assert g.number_of_nodes(ntype) == sg.number_of_nodes(ntype)

        assert F.array_equal(F.tensor(sg.edges['follows'].data[dgl.EID]),
                             F.tensor([1], g.idtype))

        if not preserve_nodes:
            assert F.array_equal(sg.nodes['user'].data['h'], g.nodes['user'].data['h'][1:3])
        assert F.array_equal(sg.edges['follows'].data['h'], g.edges['follows'].data['h'][1:2])

    def _check_subgraph_single_etype(g, sg, preserve_nodes=False):
        assert sg.ntypes == g.ntypes
        assert sg.etypes == g.etypes
        assert sg.canonical_etypes == g.canonical_etypes

        if not preserve_nodes:
            assert F.array_equal(F.tensor(sg.nodes['user'].data[dgl.NID]),
                                 F.tensor([0, 1], g.idtype))
            assert F.array_equal(F.tensor(sg.nodes['game'].data[dgl.NID]),
                                 F.tensor([0], g.idtype))
        else:
            for ntype in sg.ntypes:
                assert g.number_of_nodes(ntype) == sg.number_of_nodes(ntype)

        assert F.array_equal(F.tensor(sg.edges['plays'].data[dgl.EID]),
                             F.tensor([0, 1], g.idtype))

    sg1_graph = g_graph.subgraph([1, 2])
    _check_subgraph_single_ntype(g_graph, sg1_graph)
    sg1_graph = g_graph.edge_subgraph([1])
    _check_subgraph_single_ntype(g_graph, sg1_graph)
    sg1_graph = g_graph.edge_subgraph([1], relabel_nodes=False)
    _check_subgraph_single_ntype(g_graph, sg1_graph, True)
    sg2_bipartite = g_bipartite.edge_subgraph([0, 1])
    _check_subgraph_single_etype(g_bipartite, sg2_bipartite)
    sg2_bipartite = g_bipartite.edge_subgraph([0, 1], relabel_nodes=False)
    _check_subgraph_single_etype(g_bipartite, sg2_bipartite, True)

    def _check_typed_subgraph1(g, sg):
        assert g.idtype == sg.idtype
        assert g.device == sg.device
        assert set(sg.ntypes) == {'user', 'game'}
        assert set(sg.etypes) == {'follows', 'plays', 'wishes'}
        for ntype in sg.ntypes:
            assert sg.number_of_nodes(ntype) == g.number_of_nodes(ntype)
        for etype in sg.etypes:
            src_sg, dst_sg = sg.all_edges(etype=etype, order='eid')
            src_g, dst_g = g.all_edges(etype=etype, order='eid')
            assert F.array_equal(src_sg, src_g)
            assert F.array_equal(dst_sg, dst_g)
        assert F.array_equal(sg.nodes['user'].data['h'], g.nodes['user'].data['h'])
        assert F.array_equal(sg.edges['follows'].data['h'], g.edges['follows'].data['h'])
        g.nodes['user'].data['h'] = F.scatter_row(g.nodes['user'].data['h'], F.tensor([2]), F.randn((1, 5)))
        g.edges['follows'].data['h'] = F.scatter_row(g.edges['follows'].data['h'], F.tensor([1]), F.randn((1, 4)))
        assert F.array_equal(sg.nodes['user'].data['h'], g.nodes['user'].data['h'])
        assert F.array_equal(sg.edges['follows'].data['h'], g.edges['follows'].data['h'])

    def _check_typed_subgraph2(g, sg):
        assert set(sg.ntypes) == {'developer', 'game'}
        assert set(sg.etypes) == {'develops'}
        for ntype in sg.ntypes:
            assert sg.number_of_nodes(ntype) == g.number_of_nodes(ntype)
        for etype in sg.etypes:
            src_sg, dst_sg = sg.all_edges(etype=etype, order='eid')
            src_g, dst_g = g.all_edges(etype=etype, order='eid')
            assert F.array_equal(src_sg, src_g)
            assert F.array_equal(dst_sg, dst_g)

    sg3 = g.node_type_subgraph(['user', 'game'])
    _check_typed_subgraph1(g, sg3)
    sg4 = g.edge_type_subgraph(['develops'])
    _check_typed_subgraph2(g, sg4)
    sg5 = g.edge_type_subgraph(['follows', 'plays', 'wishes'])
    _check_typed_subgraph1(g, sg5)

    # Test for restricted format
    for fmt in ['csr', 'csc', 'coo']:
        g = dgl.graph(([0, 1], [1, 2])).formats(fmt)
        sg = g.subgraph({g.ntypes[0]: [1, 0]})
        nids = F.asnumpy(sg.ndata[dgl.NID])
        assert np.array_equal(nids, np.array([1, 0]))
        src, dst = sg.edges(order='eid')
        src = F.asnumpy(src)
        dst = F.asnumpy(dst)
        assert np.array_equal(src, np.array([1]))

@parametrize_idtype
def test_in_subgraph(idtype):
    hg = dgl.heterograph({
        ('user', 'follow', 'user'): ([1, 2, 3, 0, 2, 3, 0], [0, 0, 0, 1, 1, 1, 2]),
        ('user', 'play', 'game'): ([0, 0, 1, 3], [0, 1, 2, 2]),
        ('game', 'liked-by', 'user'): ([2, 2, 2, 1, 1, 0], [0, 1, 2, 0, 3, 0]),
        ('user', 'flips', 'coin'): ([0, 1, 2, 3], [0, 0, 0, 0])
    }, idtype=idtype, num_nodes_dict={'user': 5, 'game': 10, 'coin': 8}).to(F.ctx())
    subg = dgl.in_subgraph(hg, {'user' : [0,1], 'game' : 0})
    assert subg.idtype == idtype
    assert len(subg.ntypes) == 3
    assert len(subg.etypes) == 4
    u, v = subg['follow'].edges()
    edge_set = set(zip(list(F.asnumpy(u)), list(F.asnumpy(v))))
    assert F.array_equal(hg['follow'].edge_ids(u, v), subg['follow'].edata[dgl.EID])
    assert edge_set == {(1,0),(2,0),(3,0),(0,1),(2,1),(3,1)}
    u, v = subg['play'].edges()
    edge_set = set(zip(list(F.asnumpy(u)), list(F.asnumpy(v))))
    assert F.array_equal(hg['play'].edge_ids(u, v), subg['play'].edata[dgl.EID])
    assert edge_set == {(0,0)}
    u, v = subg['liked-by'].edges()
    edge_set = set(zip(list(F.asnumpy(u)), list(F.asnumpy(v))))
    assert F.array_equal(hg['liked-by'].edge_ids(u, v), subg['liked-by'].edata[dgl.EID])
    assert edge_set == {(2,0),(2,1),(1,0),(0,0)}
    assert subg['flips'].number_of_edges() == 0
    for ntype in subg.ntypes:
        assert dgl.NID not in subg.nodes[ntype].data

    # Test store_ids
    subg = dgl.in_subgraph(hg, {'user': [0, 1], 'game': 0}, store_ids=False)
    for etype in ['follow', 'play', 'liked-by']:
        assert dgl.EID not in subg.edges[etype].data
    for ntype in subg.ntypes:
        assert dgl.NID not in subg.nodes[ntype].data

    # Test relabel nodes
    subg = dgl.in_subgraph(hg, {'user': [0, 1], 'game': 0}, relabel_nodes=True)
    assert subg.idtype == idtype
    assert len(subg.ntypes) == 3
    assert len(subg.etypes) == 4

    u, v = subg['follow'].edges()
    old_u = F.gather_row(subg.nodes['user'].data[dgl.NID], u)
    old_v = F.gather_row(subg.nodes['user'].data[dgl.NID], v)
    assert F.array_equal(hg['follow'].edge_ids(old_u, old_v), subg['follow'].edata[dgl.EID])
    edge_set = set(zip(list(F.asnumpy(old_u)), list(F.asnumpy(old_v))))
    assert edge_set == {(1,0),(2,0),(3,0),(0,1),(2,1),(3,1)}

    u, v = subg['play'].edges()
    old_u = F.gather_row(subg.nodes['user'].data[dgl.NID], u)
    old_v = F.gather_row(subg.nodes['game'].data[dgl.NID], v)
    assert F.array_equal(hg['play'].edge_ids(old_u, old_v), subg['play'].edata[dgl.EID])
    edge_set = set(zip(list(F.asnumpy(old_u)), list(F.asnumpy(old_v))))
    assert edge_set == {(0,0)}

    u, v = subg['liked-by'].edges()
    old_u = F.gather_row(subg.nodes['game'].data[dgl.NID], u)
    old_v = F.gather_row(subg.nodes['user'].data[dgl.NID], v)
    assert F.array_equal(hg['liked-by'].edge_ids(old_u, old_v), subg['liked-by'].edata[dgl.EID])
    edge_set = set(zip(list(F.asnumpy(old_u)), list(F.asnumpy(old_v))))
    assert edge_set == {(2,0),(2,1),(1,0),(0,0)}

    assert subg.num_nodes('user') == 4
    assert subg.num_nodes('game') == 3
    assert subg.num_nodes('coin') == 0
    assert subg.num_edges('flips') == 0

@parametrize_idtype
def test_out_subgraph(idtype):
    hg = dgl.heterograph({
        ('user', 'follow', 'user'): ([1, 2, 3, 0, 2, 3, 0], [0, 0, 0, 1, 1, 1, 2]),
        ('user', 'play', 'game'): ([0, 0, 1, 3], [0, 1, 2, 2]),
        ('game', 'liked-by', 'user'): ([2, 2, 2, 1, 1, 0], [0, 1, 2, 0, 3, 0]),
        ('user', 'flips', 'coin'): ([0, 1, 2, 3], [0, 0, 0, 0])
    }, idtype=idtype).to(F.ctx())
    subg = dgl.out_subgraph(hg, {'user' : [0,1], 'game' : 0})
    assert subg.idtype == idtype
    assert len(subg.ntypes) == 3
    assert len(subg.etypes) == 4
    u, v = subg['follow'].edges()
    edge_set = set(zip(list(F.asnumpy(u)), list(F.asnumpy(v))))
    assert edge_set == {(1,0),(0,1),(0,2)}
    assert F.array_equal(hg['follow'].edge_ids(u, v), subg['follow'].edata[dgl.EID])
    u, v = subg['play'].edges()
    edge_set = set(zip(list(F.asnumpy(u)), list(F.asnumpy(v))))
    assert edge_set == {(0,0),(0,1),(1,2)}
    assert F.array_equal(hg['play'].edge_ids(u, v), subg['play'].edata[dgl.EID])
    u, v = subg['liked-by'].edges()
    edge_set = set(zip(list(F.asnumpy(u)), list(F.asnumpy(v))))
    assert edge_set == {(0,0)}
    assert F.array_equal(hg['liked-by'].edge_ids(u, v), subg['liked-by'].edata[dgl.EID])
    u, v = subg['flips'].edges()
    edge_set = set(zip(list(F.asnumpy(u)), list(F.asnumpy(v))))
    assert edge_set == {(0,0),(1,0)}
    assert F.array_equal(hg['flips'].edge_ids(u, v), subg['flips'].edata[dgl.EID])
    for ntype in subg.ntypes:
        assert dgl.NID not in subg.nodes[ntype].data

    # Test store_ids
    subg = dgl.out_subgraph(hg, {'user' : [0,1], 'game' : 0}, store_ids=False)
    for etype in subg.canonical_etypes:
        assert dgl.EID not in subg.edges[etype].data
    for ntype in subg.ntypes:
        assert dgl.NID not in subg.nodes[ntype].data

    # Test relabel nodes
    subg = dgl.out_subgraph(hg, {'user': [1], 'game': 0}, relabel_nodes=True)
    assert subg.idtype == idtype
    assert len(subg.ntypes) == 3
    assert len(subg.etypes) == 4

    u, v = subg['follow'].edges()
    old_u = F.gather_row(subg.nodes['user'].data[dgl.NID], u)
    old_v = F.gather_row(subg.nodes['user'].data[dgl.NID], v)
    edge_set = set(zip(list(F.asnumpy(old_u)), list(F.asnumpy(old_v))))
    assert edge_set == {(1, 0)}
    assert F.array_equal(hg['follow'].edge_ids(old_u, old_v), subg['follow'].edata[dgl.EID])

    u, v = subg['play'].edges()
    old_u = F.gather_row(subg.nodes['user'].data[dgl.NID], u)
    old_v = F.gather_row(subg.nodes['game'].data[dgl.NID], v)
    edge_set = set(zip(list(F.asnumpy(old_u)), list(F.asnumpy(old_v))))
    assert edge_set == {(1, 2)}
    assert F.array_equal(hg['play'].edge_ids(old_u, old_v), subg['play'].edata[dgl.EID])

    u, v = subg['liked-by'].edges()
    old_u = F.gather_row(subg.nodes['game'].data[dgl.NID], u)
    old_v = F.gather_row(subg.nodes['user'].data[dgl.NID], v)
    edge_set = set(zip(list(F.asnumpy(old_u)), list(F.asnumpy(old_v))))
    assert edge_set == {(0,0)}
    assert F.array_equal(hg['liked-by'].edge_ids(old_u, old_v), subg['liked-by'].edata[dgl.EID])

    u, v = subg['flips'].edges()
    old_u = F.gather_row(subg.nodes['user'].data[dgl.NID], u)
    old_v = F.gather_row(subg.nodes['coin'].data[dgl.NID], v)
    edge_set = set(zip(list(F.asnumpy(old_u)), list(F.asnumpy(old_v))))
    assert edge_set == {(1,0)}
    assert F.array_equal(hg['flips'].edge_ids(old_u, old_v), subg['flips'].edata[dgl.EID])
    assert subg.num_nodes('user') == 2
    assert subg.num_nodes('game') == 2
    assert subg.num_nodes('coin') == 1

def test_subgraph_message_passing():
    # Unit test for PR #2055
    g = dgl.graph(([0, 1, 2], [2, 3, 4])).to(F.cpu())
    g.ndata['x'] = F.copy_to(F.randn((5, 6)), F.cpu())
    sg = g.subgraph([1, 2, 3]).to(F.ctx())
    sg.update_all(lambda edges: {'x': edges.src['x']}, lambda nodes: {'y': F.sum(nodes.mailbox['x'], 1)})

@parametrize_idtype
def test_khop_in_subgraph(idtype):
    g = dgl.graph(([1, 1, 2, 3, 4], [0, 2, 0, 4, 2]), idtype=idtype, device=F.ctx())
    g.edata['w'] = F.tensor([
        [0, 1],
        [2, 3],
        [4, 5],
        [6, 7],
        [8, 9]
    ])
    sg, inv = dgl.khop_in_subgraph(g, 0, k=2)
    assert sg.idtype == g.idtype
    u, v = sg.edges()
    edge_set = set(zip(list(F.asnumpy(u)), list(F.asnumpy(v))))
    assert edge_set == {(1,0), (1,2), (2,0), (3,2)}
    assert F.array_equal(sg.edata[dgl.EID], F.tensor([0, 1, 2, 4], dtype=idtype))
    assert F.array_equal(sg.edata['w'], F.tensor([
        [0, 1],
        [2, 3],
        [4, 5],
        [8, 9]
    ]))
    assert F.array_equal(F.astype(inv, idtype), F.tensor([0], idtype))

    # Test multiple nodes
    sg, inv = dgl.khop_in_subgraph(g, [0, 2], k=1)
    assert sg.num_edges() == 4

    sg, inv = dgl.khop_in_subgraph(g, F.tensor([0, 2], idtype), k=1)
    assert sg.num_edges() == 4

    # Test isolated node
    sg, inv = dgl.khop_in_subgraph(g, 1, k=2)
    assert sg.idtype == g.idtype
    assert sg.num_nodes() == 1
    assert sg.num_edges() == 0
    assert F.array_equal(F.astype(inv, idtype), F.tensor([0], idtype))

    g = dgl.heterograph({
        ('user', 'plays', 'game'): ([0, 1, 1, 2], [0, 0, 2, 1]),
        ('user', 'follows', 'user'): ([0, 1, 1], [1, 2, 2]),
    }, idtype=idtype, device=F.ctx())
    sg, inv = dgl.khop_in_subgraph(g, {'game': 0}, k=2)
    assert sg.idtype == idtype
    assert sg.num_nodes('game') == 1
    assert sg.num_nodes('user') == 2
    assert len(sg.ntypes) == 2
    assert len(sg.etypes) == 2
    u, v = sg['follows'].edges()
    edge_set = set(zip(list(F.asnumpy(u)), list(F.asnumpy(v))))
    assert edge_set == {(0, 1)}
    u, v = sg['plays'].edges()
    edge_set = set(zip(list(F.asnumpy(u)), list(F.asnumpy(v))))
    assert edge_set == {(0, 0), (1, 0)}
    assert F.array_equal(F.astype(inv['game'], idtype), F.tensor([0], idtype))

    # Test isolated node
    sg, inv = dgl.khop_in_subgraph(g, {'user': 0}, k=2)
    assert sg.idtype == idtype
    assert sg.num_nodes('game') == 0
    assert sg.num_nodes('user') == 1
    assert sg.num_edges('follows') == 0
    assert sg.num_edges('plays') == 0
    assert F.array_equal(F.astype(inv['user'], idtype), F.tensor([0], idtype))

    # Test multiple nodes
    sg, inv = dgl.khop_in_subgraph(g, {'user': F.tensor([0, 1], idtype), 'game': 0}, k=1)
    u, v = sg['follows'].edges()
    edge_set = set(zip(list(F.asnumpy(u)), list(F.asnumpy(v))))
    assert edge_set == {(0, 1)}
    u, v = sg['plays'].edges()
    edge_set = set(zip(list(F.asnumpy(u)), list(F.asnumpy(v))))
    assert edge_set == {(0, 0), (1, 0)}
    assert F.array_equal(F.astype(inv['user'], idtype), F.tensor([0, 1], idtype))
    assert F.array_equal(F.astype(inv['game'], idtype), F.tensor([0], idtype))

@parametrize_idtype
def test_khop_out_subgraph(idtype):
    g = dgl.graph(([0, 2, 0, 4, 2], [1, 1, 2, 3, 4]), idtype=idtype, device=F.ctx())
    g.edata['w'] = F.tensor([
        [0, 1],
        [2, 3],
        [4, 5],
        [6, 7],
        [8, 9]
    ])
    sg, inv = dgl.khop_out_subgraph(g, 0, k=2)
    assert sg.idtype == g.idtype
    u, v = sg.edges()
    edge_set = set(zip(list(F.asnumpy(u)), list(F.asnumpy(v))))
    assert edge_set == {(0,1), (2,1), (0,2), (2,3)}
    assert F.array_equal(sg.edata[dgl.EID], F.tensor([0, 2, 1, 4], dtype=idtype))
    assert F.array_equal(sg.edata['w'], F.tensor([
        [0, 1],
        [4, 5],
        [2, 3],
        [8, 9]
    ]))
    assert F.array_equal(F.astype(inv, idtype), F.tensor([0], idtype))

    # Test multiple nodes
    sg, inv = dgl.khop_out_subgraph(g, [0, 2], k=1)
    assert sg.num_edges() == 4

    sg, inv = dgl.khop_out_subgraph(g, F.tensor([0, 2], idtype), k=1)
    assert sg.num_edges() == 4

    # Test isolated node
    sg, inv = dgl.khop_out_subgraph(g, 1, k=2)
    assert sg.idtype == g.idtype
    assert sg.num_nodes() == 1
    assert sg.num_edges() == 0
    assert F.array_equal(F.astype(inv, idtype), F.tensor([0], idtype))

    g = dgl.heterograph({
        ('user', 'plays', 'game'): ([0, 1, 1, 2], [0, 0, 2, 1]),
        ('user', 'follows', 'user'): ([0, 1], [1, 3]),
    }, idtype=idtype, device=F.ctx())
    sg, inv = dgl.khop_out_subgraph(g, {'user': 0}, k=2)
    assert sg.idtype == idtype
    assert sg.num_nodes('game') == 2
    assert sg.num_nodes('user') == 3
    assert len(sg.ntypes) == 2
    assert len(sg.etypes) == 2
    u, v = sg['follows'].edges()
    edge_set = set(zip(list(F.asnumpy(u)), list(F.asnumpy(v))))
    assert edge_set == {(0, 1), (1, 2)}
    u, v = sg['plays'].edges()
    edge_set = set(zip(list(F.asnumpy(u)), list(F.asnumpy(v))))
    assert edge_set == {(0,0), (1,0), (1,1)}
    assert F.array_equal(F.astype(inv['user'], idtype), F.tensor([0], idtype))

    # Test isolated node
    sg, inv = dgl.khop_out_subgraph(g, {'user': 3}, k=2)
    assert sg.idtype == idtype
    assert sg.num_nodes('game') == 0
    assert sg.num_nodes('user') == 1
    assert sg.num_edges('follows') == 0
    assert sg.num_edges('plays') == 0
    assert F.array_equal(F.astype(inv['user'], idtype), F.tensor([0], idtype))

    # Test multiple nodes
    sg, inv = dgl.khop_out_subgraph(g, {'user': F.tensor([2], idtype), 'game': 0}, k=1)
    assert sg.num_edges('follows') == 0
    u, v = sg['plays'].edges()
    edge_set = set(zip(list(F.asnumpy(u)), list(F.asnumpy(v))))
    assert edge_set == {(0, 1)}
    assert F.array_equal(F.astype(inv['user'], idtype), F.tensor([0], idtype))
    assert F.array_equal(F.astype(inv['game'], idtype), F.tensor([0], idtype))

@unittest.skipIf(not F.gpu_ctx(), 'only necessary with GPU')
@pytest.mark.parametrize(
    'parent_idx_device', [('cpu', F.cpu()), ('cuda', F.cuda()), ('uva', F.cpu()), ('uva', F.cuda())])
@pytest.mark.parametrize('child_device', [F.cpu(), F.cuda()])
def test_subframes(parent_idx_device, child_device):
    parent_device, idx_device = parent_idx_device
    g = dgl.graph((F.tensor([1,2,3], dtype=F.int64), F.tensor([2,3,4], dtype=F.int64)))
    print(g.device)
    g.ndata['x'] = F.randn((5, 4))
    g.edata['a'] = F.randn((3, 6))
    idx = F.tensor([1, 2], dtype=F.int64)
    if parent_device == 'cuda':
        g = g.to(F.cuda())
    elif parent_device == 'uva':
        if F.backend_name != 'pytorch':
            pytest.skip("UVA only supported for PyTorch")
        g = g.to(F.cpu())
        g.create_formats_()
        g.pin_memory_()
    elif parent_device == 'cpu':
        g = g.to(F.cpu())
    idx = F.copy_to(idx, idx_device)
    sg = g.sample_neighbors(idx, 2).to(child_device)
    assert sg.device == F.context(sg.ndata['x'])
    assert sg.device == F.context(sg.edata['a'])
    assert sg.device == child_device
    if parent_device != 'uva':
        sg = g.to(child_device).sample_neighbors(F.copy_to(idx, child_device), 2)
        assert sg.device == F.context(sg.ndata['x'])
        assert sg.device == F.context(sg.edata['a'])
        assert sg.device == child_device
    if parent_device == 'uva':
        g.unpin_memory_()

@unittest.skipIf(F._default_context_str != "gpu", reason="UVA only available on GPU")
@pytest.mark.parametrize('device', [F.cpu(), F.cuda()])
@unittest.skipIf(dgl.backend.backend_name != "pytorch", reason="UVA only supported for PyTorch")
@parametrize_idtype
def test_uva_subgraph(idtype, device):
    g = create_test_heterograph(idtype)
    g = g.to(F.cpu())
    g.create_formats_()
    g.pin_memory_()
    indices = {'user': F.copy_to(F.tensor([0], idtype), device)}
    edge_indices = {'follows': F.copy_to(F.tensor([0], idtype), device)}
    assert g.subgraph(indices).device == device
    assert g.edge_subgraph(edge_indices).device == device
    assert g.in_subgraph(indices).device == device
    assert g.out_subgraph(indices).device == device
    if dgl.backend.backend_name != 'tensorflow':
        # (BarclayII) Most of Tensorflow functions somehow do not preserve device: a CPU tensor
        # becomes a GPU tensor after operations such as concat(), unique() or even sin().
        # Not sure what should be the best fix.
        assert g.khop_in_subgraph(indices, 1)[0].device == device
        assert g.khop_out_subgraph(indices, 1)[0].device == device
    assert g.sample_neighbors(indices, 1).device == device
    g.unpin_memory_()

if __name__ == '__main__':
    test_edge_subgraph()
    # test_uva_subgraph(F.int64, F.cpu())